The recognized need for sulfur oxide removal from flue gas has been known since the early 1900's when the large volumes of SOx emissions from power plants were first noted to cause an impact on the environment. Since then the field has grown substantially with the modern era of flue gas desulfurization (FGD) being implemented on a commercial scale since the early 1970s. While many methods exist for the removal of this acid gas from flue gas the most prevalent is a calcium-based FGD process including limestone-based Wet FGD and lime-based Semi-dry FGD. In these respective methods, the SOx containing flue gas is contacted with a slurry or wet powder based spray in an absorption tower. For instance, the acid gas is absorbed into the liquid and reacts with the base to form a neutral insoluble calcium salt. The salt is removed by filtration and disposed of or sold as a byproduct.
The pressure on utilities to continuously reduce emissions from power plants, most particularly coal-fired plants, has continuously increased adoption of FGD technology including both new construction and retrofit applications. While effective, the FGD technology must be improved further to meet more restricted emission standards and to reduce the capital and operating cost. Some of the legislation passed in the US recently includes the interstate transport rule which would require by the EPA's own estimate the further installation of pollution control devices or a switch to lower sulfur coal. Under current prevailing market conditions this would instead mean a switch to natural gas. The pressure to reduce emissions is also strengthening in China where published rules would require SOx removal from coal flue gas (less than 50 μm/m3, e.g. 17.5 ppm in volume equivalent) beyond the design limits of state-of-the-art FGD systems to fight the serious smog in the eastern coast of China.
The state-of-art calcium-based FGD technology has been increased with modern design capabilities stated at as high as 99% SOx removal. However, the bulk of wet systems operate at around 97% capture efficiency with the absolute SO2 emission level of no less than 50 ppm typically due to the variation of unit loading, ambient conditions and FGD downtime. If the new Chinese emission standard is set at 17.5 ppm, for a flue gas containing 4000 ppm produced from a coal with sulfur content of approximately 3.5%, a minimum removal efficiency of 99.6% will be required meaning that new concepts in flue gas desulfurization will need to be implemented while maintaining the cost-effectiveness of a calcium-based FGD system. Here, a new concept in FGD is proposed that integrates a traditional wet calcium FGD (Ca-WGD) for coarse removal with an additional sodium-based packed bed absorber (Na-PBA) at the top for deep SOx capture precipitated by addition of calcium to the solution. The integrated process maximizes efficiency and minimizes cost. The unit design also prevents fouling, scaling, or plugging of the packed bed absorber that would be associated with the implementation of packing above a traditional Ca-WFGD.